How the British Royals hid the crown jewels from the Nazis

The Queen's crown jewels were hidden in a biscuit tin underground at Windsor Castle during World War II, according to a BBC documentary.

The operation was an effort to keep the precious gems out of Nazi hands.

The Queen only learned of the operation — which happened when she was 14 — while filming the documentary.

It's well known that the crown jewels — a collection of crowns, robes, and other regalia kept at the Tower of London for over 600 years — are incredibly precious.

So much so that some gems from the collection were hidden underground in a biscuit tin at Windsor Castle during World War II to keep them away from the Nazis, according to a BBC documentary.

The precious stones were placed in a Bath Oliver tin in a position where the grass could regrow and conceal the hiding place, according to The Times.

The operation was so secret that even the Queen didn't know what had happened until the filming of a new BBC documentary about the Coronation.

BBC/ITV Archive

Royal commentator reportedly Alastair Bruce discovered the story among "confidential correspondence in the Royal Collection."

This correspondence was an "electric set of letters" from royal librarian Sir Owen Morshead to Queen Mary, the mother of George VI. The hiding of the precious gems was ordered by George VI.

In order to hide the precious cargo, a "deep hole" was dug in the grounds "beneath a sally port, one of the secure entries to the castle, and two chambers constructed with steel doors," according to The Times.

The work had to be covered up at night. "They dug out this fresh, very virgin white chalk and they had to hide it with tarpaulins so when the aircraft flew over at night no clue was given to the German Luftwaffe that anything was going on," Bruce wrote.

The Jewels were then locked inside, only accessible through a trapdoor, which still exists.

The most precious jewels — the Black Prince's Ruby and St Edward's Sapphire — were even removed from the Imperial State Crown and kept separately in the biscuit tin "in case of emergency."

Speaking to the Queen for the BBC documentary, Bruce told Her Majesty — who had been only 14 at the time — the story.

"What was so lovely was that the Queen had no knowledge of it," he said. "Telling her seemed strangely odd."

He added that Her Majesty had been aware that the jewels were hidden at Windsor by 1940, when the government was trying to hide stocks of water, but had no idea where they were buried — or that they were hiding in a biscuit tin.

The tell-all documentary will see the Queen talking about her Coronation, including what it's really like to wear a heavy crown and robes.

She reportedly jokes that you "cannot look down" while wearing the 2lbs, 13oz Imperial State Crown or you neck would "break."